Effective means something achieves the result it was meant to achieve: "the new treatment is effective against the infection." Efficient means it achieves that result with little waste of time, money or energy: "the new boiler is far more efficient." Effective asks whether it worked; efficient asks what it cost to work.
The core difference
Effectiveness is about outcome. Efficiency is about ratio — output relative to input. That is why the two can come apart in either direction, and why muddling them causes real confusion in business and technical writing.
Using effective
Use effective when the question is whether the thing does its job at all.
- Ear defenders are the most effective protection on a building site.
- Her argument was effective because it named specific numbers.
- The vaccine is 90 per cent effective against severe illness.
Using efficient
Use efficient when the question is how much is consumed along the way.
- The engine is 30 per cent more fuel-efficient than the previous model.
- Splitting the file first is a more efficient way to process the data.
- He is a quiet, efficient administrator — nothing takes twice as long as it should.
The test
Ask "did it work?" If that is what you mean, write effective. Ask "at what cost?" If that is the point, write efficient. As a cross-check, efficient nearly always invites a resource to be named — fuel, time, cost, memory, staff hours. If no resource is in play, effective is almost certainly your word.
Why the distinction matters
Something can be effective but inefficient: hand-delivering a hundred letters gets them all there and wastes a day. Something can be efficient but ineffective: an email campaign that costs nothing per recipient and persuades none of them. And a process can be beautifully efficient at doing entirely the wrong thing, which is the failure mode the distinction exists to expose.
In writing about performance, state which one you mean. "We've improved our recruitment" tells the reader nothing; "we fill roles 40 per cent faster" is efficiency, and "our new hires stay twice as long" is effectiveness.
The edge cases
Effective has a second, unrelated sense meaning in force from: "the ban is effective from 1 April," "the effective date of the policy." Efficient never does this.
There is also effectively, which has drifted. It can mean "in an effective manner" ("she argued effectively") or "in practice, if not officially" ("he is effectively in charge"). The second sense is now much commoner, so if you mean the first, consider rewording to avoid ambiguity.
Finally, efficacious and efficacy belong to the effective family, not the efficient one, despite the spelling. They are chiefly medical: a drug's efficacy is how well it works under ideal trial conditions.
Frequently Asked Questions
Can something be efficient but not effective?
Yes, and it is a common trap in organisations. A process that completes quickly and cheaply but fails to produce the intended result is efficient and ineffective — doing the wrong thing well. Effectiveness should be settled first; efficiency is how you then improve it.
Is 'cost-effective' the same as 'efficient'?
They are close but not identical. Cost-effective means the result justifies the money spent — it is a judgement about value. Efficient is a broader ratio of output to any input, including time, fuel or effort, not just money.